The expense of MOD files, along with all similar formats, i.e. IT, MIDI, etc comes down to how complex the song is. There are no hard facts for this. To throw a ballpark figure I would estimate around 20% usage or more for a song playing 10 channels at once, but thats really on a guess. If you are crafting MODs for the iPhone you can optimize sample rates to prevent a lot of the overhead of resampling.

I would recommend you download the SDK, open up the playsound example and throw a few MOD files in using System::getCPUUsage() to find out the expense.